How can I remove a query parameter from a URL?

if I know that the Context_ parameter(s) will be the last parameter(s)

Try this

when HTTP_REQUEST { 
   if {[HTTP::query] contains "Context_"} {
      set pos [string first "Context_" [HTTP::query]]
      set newq [string range [HTTP::query] 0 [expr $pos -2]]
      HTTP::uri "[HTTP::path]?$newq"
   }
}